Oil country tubular goods such as tubing and casing used in the excavation of oil wells and gas wells are usually connected to each other by threaded joints for pipes. In the past, the depth of oil wells was typically 2,000-3,000 meters, but in deep oil wells such as recent offshore oil fields, it may reach 8,000-15,000 meters. In most offshore oil fields, excavation of oil wells are carried out on a floating facility, and due to rocking of the entire facility the overall excavation operation is performed in unstable conditions.
In their environment of use, threaded joint for pipes for oil country tubular goods are subjected to loads such as axial tensile forces caused by the weight of the oil country tubular goods and the threaded joint for pipes themselves (up to 500 tons in total in the case of 7 inch steel pipes), bending forces, the combination of internal and external pressures (up to 1000 atm), and geothermal heat (200° C. or above, and up to 300° C. in some locations). Accordingly, they need to be able to maintain airtightness without undergoing damage even in such a severe environment.
A typical threaded joint for pipes used for connecting oil country tubular goods is constituted by a pin, which is a joint element having a male thread formed on the end portion of a first tubular member (which is typically an oil country tubular good), and a box, which is a joint element having a female thread formed on the end portions of a second tubular member (which is typically a threaded connecting member referred to as a coupling).
As shown in FIG. 1, with a special type of threaded joint for pipes referred to as a premium joint which was developed in order to exhibit a high degree of gas-tightness even in severe environments, there is a sealing surface on the outer peripheral portion in the vicinity of the end surface closer to the end than the male threads of the pin and on the inner peripheral surface of the base portion of the female threads of the box, and the end surface of the end of the pin and the corresponding rearmost surface of the box become torque shoulders. The portions of the pin and the box including the sealing surface and the torque shoulder are referred to as unthreaded metal contact portion. This portion of the pin is also referred to as the torque shoulder or the lip portion.
The premium threaded joint is designed so that when one end (a pin) of an oil country tubular good is inserted into a coupling (a box) and after the torque shoulders of the pin and the box contact each other the male threads and the female threads are tightened until they interfere, and the sealing surfaces of the pin and the box contact with suitable interference and form a seal by metal-to-metal contact. The gas-tightness of a threaded joint is guaranteed by such a metal seal at the sealing surface.
During the process of lowering tubing or casing into an oil well, due to various problems, it is sometimes necessary to break out a joint which has been once made up, to lift the pipes out of the oil well, to remake up them, and then relower them. API (American Petroleum Institute) requires galling resistance such that galling does not occur and airtightness is maintained even if make-up (tightening) and breakout (loosening) are repeated ten times for a joint for tubing or three times for a joint for casing.
At the time of make-up, in order to increase galling resistance and airtightness, a viscous liquid lubricant (greasy lubricant) which contains heavy metal powders and which is referred to as “compound grease” is applied to the contact surfaces (namely, the threaded portions and the unthreaded metal contact portions) of a threaded joint for pipes. Such a compound grease is specified by API Bulletin 5A2. A compound grease is required to be able to impart rust prevention to the contact surface to which the compound grease was applied.
In the past, it has been proposed to carry out various types of surface treatment such as nitriding, various types of plating including zinc plating and composite plating, and phosphate chemical conversion treatment on the contact surface of a threaded joint for pipes to form one or more layers in order to increase the retention of compound grease and improve sliding properties. However, as described below, the use of compound grease poses the threat of harmful effects on the environment and humans.
Compound grease contains large amounts of powders of heavy metals such as zinc, lead, and copper. When make-up of a threaded joint for pipes is carried out, grease which has been applied is washed off or overflows to the exterior surface, and there is the possibility of its producing harmful effects on the environment and especially on sea life, particularly due to harmful heavy metals such as lead. In addition, the process of applying compound grease worsens the working environment, and there is also a concern of its having harmful effects on humans.
In recent years, as a result of the enactment in 1998 of the OSPAR Convention (Oslo-Paris Convention) for preventing ocean pollution in the Northeast Atlantic, strict restrictions concerning the global environment are becoming more numerous, and in some regions, the use of compound grease is already in the process of restriction. Accordingly, in order to avoid harmful effects on the environment and humans in the excavation of gas wells and oil wells, a demand has developed for threaded joint for pipes which can exhibit excellent galling resistance without using compound grease. In addition, the OSPAR Convention not only regulates the use of heavy metals but also requires the use of organic substances having no or little toxicity and good biodgradability.
As a threaded joint for pipes which can be used for connecting oil country tubular goods without application of compound grease, the present inventors proposed in JP 2002-173692A (Patent Document 1) a threaded joint for pipes having a viscous liquid or semisolid lubricating coating formed thereon, and in JP 2004-53013A (Patent Document 2) a threaded joint for pipes in which tackiness of the surface of the threaded joint for pipes, which is a drawback of a viscous liquid or semisolid lubricating coating, is suppressed to minimize the adhesion of foreign matter such as dust, sand, and debris.
As stated above, in a premium threaded joint having unthreaded metal contact portions in which the pin and the box respectively have a sealing surface and a torque shoulder, gas-tightness is guaranteed by forming a metal-to-metal seal between the sealing surfaces of the pin and the box at the time of make-up.
FIG. 2 shows a torque chart (vertical axis: torque, horizontal axis: turns) at the time of make-up of this type of threaded joint. As shown in this Figure, if the pin is inserted into a box and the pin (or the box) is rotated, as rotation takes place, initially primarily the threaded portions of the pin and the box contact and torque gradually increases. As rotation advances, and the sealing surfaces of the pin and the box contact each other, the rate of increase of torque increases due to frictional resistance. If rotation further progresses and the shoulder surface at the end of the pin and the shoulder surface of the box contact each other and begin to interfere (the torque at the start of this interference is called a shouldering torque Ts), interference between the sealing surfaces takes place and the torque abruptly increases. If rotation further advances from this state and the prescribed make-up torque is reached, make-up is completed. The optimal torque in FIG. 2 means the optimal torque for achieving the necessary interference for guaranteeing gas-tightness and completing make-up, and a suitable value is previously determined based on the inner diameter of the joint and the type of joint.
However, premium threaded joints used in ultra deep wells such as those exceeding 10,000 meters have an extremely high compressive stress and bending stress applied to the threaded joint, and make-up must take place with a higher than usual make-up torque (such as 120-130% of the optimal make-up torque) so that loosening does not take place. In this case, with a threaded joint for pipes having a conventional lubricating coating, the make-up torque sometimes exceeds the torque at which the unthreaded metal contact portions of the pin and the box yield and begin to undergo plastic deformation (the torque at this time being called a yielding torque Ty). As a result, both the pin and the box undergo unrecoverable damage due to the plastic deformation due to yielding of the unthreaded metal contact portion. Even if they are not damaged, the gas tightness of the threaded joint is markedly decreased. Such a phenomenon was hardly seen when a compound grease was applied.